The invention relates to a friction belt brake with a rotary cylinder which is wound around at least part of its circumference by a friction belt, and with a support for the friction belt which, in a first braking position, can be pivoted around a first axis and has two securing points for respectively opposite end sections of the friction belt. In this connection, a first of the two securing points has a longer lever arm in relation to this first axis than the second securing point.
A differentially acting friction belt brake of this kind is shown in FIG. 6. The figure depicts a rotary cylinder 1 rotating in the clockwise direction, with a friction belt 2 that is fastened to a support 3 at two securing points 10, 11. The support 3 can rotate around a fixed axis 9 disposed between the securing points; the securing points consequently define lever arms with the lengths a and b in relation to this axis 9.
The distance a+b between the securing points 10, 11 here is equal to the diameter of the rotary cylinder 1.
The brake has an equilibrium position in which the friction belt 2 is stretched tight and exerts a force F0 and F1, respectively, on the securing points 10, 11. The torques exerted by these forces on the fixed axis 9 are equal and opposite so that the support 3 does not move. A braking moment with the value (F1xe2x88x92F0)r then acts on the rotary cylinder 1, where r is the radius of the rotary cylinder and the bearing of the rotary cylinder is loaded with the sum of the forces F0+F1.
If a slight adjusting force Fs is exerted with a torque Ls on the support 3, then the friction belt 2 is stretched tighter, wherein a slight adjusting force Fs is sufficient to cause both F1 and F0 to increase sharply.
A disadvantage of this known friction belt brake is that it is not equally suited to braking rotational movements in different directions. As a result, with the friction belt brake shown in FIG. 6, the clockwise rotation of the rotary cylinder 1 results in the fact that the friction belt 2 is automatically tightened at the securing point 11 so that the above-described equilibrium position can automatically set in and be maintained without an adjusting force Fs having to be exerted in order to achieve this. If the rotary cylinder 1 were to rotate counterclockwise, then the force F0 acting on the securing point 10 would be greater than the force F1 acting on the securing point 11 and since its lever arm a is also longer than the lever arm b of the securing point 11, there is no equilibrium position. Therefore, during a counterclockwise rotation, a braking action can only be produced through the exertion of a considerable adjusting force Fs.
The invention has the advantage over the prior art that it permits an equal brake function to be produced independent of the rotation direction of the rotary cylinder to be braked. To this end, the provision is made that in the friction belt brake according to the invention, the support has two braking positions which correspond to the two possible rotation directions of the rotary cylinder, wherein in the second braking position, the support can be pivoted around a second axis in relation to which the second securing point has a longer lever arm than the first. This permits the adjustment of a stable equilibrium of the brake forces for both rotation directions.
In order to assure a symmetrical brake behavior, it is preferable that the retainer is in a position to execute a movement that is symmetrical in relation to a plane extending through the axis of the rotary cylinder. The axes of the support are preferably virtual, i.e. are not defined by means of a physical object.
The movement of the retainer from a central position into one of the two braking positions can have a multitude of rotational axes which continuously transition into one another in the course of the movement of the retainer, so-called instantaneous poles. A simple construction which permits such a movement of the support is a four-bar, i.e. a structure made up of three elements connected to one another in series, wherein the support constitutes the middle element.
In a four-bar whose lateral elements experience only tensile stress, these lateral elements can be constituted in a particularly simple way by means of end sections of the friction belt.
In such an instance, the securing points can be simple pins around which the friction belt is immovably wound. When a friction belt with inherent rigidity is used, the immobility can be simply produced by virtue of the fact that in the points at which the friction belt touches the pins, it has a curvature when unstressed which comes to rest against a part of the surface of the pin.
According to a particularly preferred embodiment, the distance of the securing points from one another is smaller than the diameter of the rotary cylinder. This leads to the fact that the friction belt 2 can come into contact with the rotary cylinder over more than half of its circumference, i.e. there is a large available contact surface on which a powerful friction can be produced. Another important advantage is that with a construction of this kind, the forces F1, F0 acting on the two securing points are no longer parallel, but have opposite signed components in the direction of the connection between the two securing points. These components compensate for each other. The force transmitted by the friction belt onto the bearing of the rotary cylinder is therefore less than the sum of the amounts of the two forces F1 and F0; as in the above-described conventional friction belt brake, however, the brake force is proportional to the difference of the amounts of these two forces. Since the winding angle of the friction belt, i.e. the portion of the circumference of the rotary cylinder that is contacted by the friction belt, is also increased, less tension of the friction belt is required in order to produce a given braking moment. This means that with the same braking moment, the bearing of the rotary cylinder is less stressed than in the conventional construction and that the friction belt brake according to the invention permits a higher braking moment to be exerted without changing the construction of the rotary cylinder bearing.
A partial compensation of the forces F0, F1 acting on the securing points would also be produced if their distance were greater than the diameter of the rotary cylinder. In such an instance, however, the contact length of the friction belt with the rotary cylinder is reduced so that relatively high forces F0, F1 are required in order to produce a given braking moment and the bearing of the rotary cylinder is powerfully stressed despite the partial compensation.
In a preferred construction of the friction belt, the provision is made that the support has an engaging point for an adjusting force acting essentially in the circumference direction of the rotary cylinder, which is disposed on the opposite side of the securing points in the radial direction with regard to the rotary cylinder. The two securing points and the engaging point define the corners of a triangle.
An annular sector is suitably disposed so that it encloses the rotary cylinder and can rotate around it. The friction belt rests against its inner wall in a rest position in which it does not touch the rotary cylinder and does not exert any braking force. The annular sector thus serves on the one hand to determine the rest position of the friction belt and prevents this belt from coming into contact with the rotary cylinder provided that the friction belt brake is not disposed in the braking position, and on the other hand, it protects the friction belt from external corrosion.
This annular sector preferably has a gearing which is engaged by a gear or a worm with which the retainer can be moved into the first or second braking position.
In this connection, the engaging point can be a hole in the support in which a guide pin connected to the annular sector engages.
The friction belt brake according to the invention is particularly suited for use in a planetary gear, in particular in a two-stage planetary gear of a starter generator for a motor vehicle.
Other features and advantages of the invention ensue from the description of exemplary embodiments below.